heres the thing...all boots are going to pack out. liners are made of an eva foam and when you try them on in the shop that is the tightest they will ever feel. you always want to try boots on before you buy, and you want them to be snug in the shop. i work at a shop and fit people for boots all the time and this is what i tell them...when you put on a brand new pair of boots you want a nice snug fit all around with no pressure points. lace up the liner and the boot entirely and then stand up in the boot. when standing up straight, you want your toes just to be grazing the front of the liner. if your toes are curled at the ends it is too small, and if you cant feel the front then they are too big. you want them just to be touching because once you flex into the boot like you would be in a snowboard stance, it will suck you into the heel pocket and take you off the front just enough so that it doesnt put pressure on your toes while riding. the reason for this is that all boots have about 15 degrees of forward lean because nobody rides standing straight up and stiff legged. the first initial days riding the boots will pack out 1/4 to 1/2 size, which is generally why a lot of people downsize their boots from their regular shoe size about 1/2 to 1 full size. try fitting into an 11 and think about the guidelines for what i told you and see how that works and remember that they will pack out.
another good thing in today's boots is that most liners are heat moldable, so if it fits well in most areas and you have minor pressure points in certain areas, you can have the shop heat the liners up and when you stick your feet in them, the liner will essentially mold to the shape of your foot, and once the foam cools down it will stay molded in that way. you can remold them too if necessary but its kind of a diminishing effect after a few times.
i also always tell customers that if it fits well out of the box but just a little snug to let the boots naturally pack out over the first couple times riding. if some pressure points become apparent while riding, bring them back into the shop and im sure they can help you out with that.
the only aftermarket liners ive heard of are from remind insoles. other than them, which their full liner is brand new, i havent heard of aftermarket liners. try looking into the aftermarket insoles too. some may be thicker than the insole in your boot and can take up some of that extra space! but the liner isn't really what makes the boots stiff, its the outer shell that makes the boot stiff.
i hear the zoom ites are very comfy and whats cool about them is they come with what i assume is a couple pieces of plastic, similar to what is in the burton driver x, that can be inserted in the tounge to increase the stiffness, so id assume that the zoom ites themselves are on the softer side, and the tongue inserts stiffen up the boot.
i hope that helps man and let me know if you have any other questions!